The present invention relates to switching power supplies, and more particularly to start-up circuitry for switching power supplies.
Switching power supplies are preferred over other types of power supplies in many applications primarily because of their small size and high efficiency operation. In designing such switching power supplies, care must be taken to prevent overvoltage conditions and overcurrent conditions which may damage circuitry of the switching power supply unless corrective action is taken. Overvoltage conditions may be detected by voltage sensing circuitry, which when activated disables power supply operation permanently by severing fusible lines or temporarily by shutting down until the over voltage condition disappears. To prevent excessive dissipation of power supply devices, circuitry may be included for detecting overcurrent conditions and likewise temporarily shutting down until the overcurrent condition disappears. Upon detection of overvoltage or overcurrent conditions, switching power supplies are disabled by limiting the current delivered to the load. Because of the presence of overvoltage and overcurrent detecting circuitry, most switching power supplies limit the current that can be delivered to the load initially at turn-on of, and after interruption of, the power source. Since the current is limited, the time required to reach the rated DC output voltage will be relatively long when the power supply is subject to heavy capacitive loads which may exceed full load capability.
Accordingly, it is an object of the present invention to provide an improved switching power supply that under momentary conditions exceeding rated full load conditions, will provide the rated DC output voltage in a relatively short period of time after initial turn-on or subsequent interruptions of the power source.